Dynamoelectric machines such as motors typically include a start or auxiliary winding and a run winding. The start or auxiliary winding is utilized to initiate rotation of the motor rotor. Particularly, the magnetic field generated by the relatively high inductive reactance start winding in a resistance split phase motor may be about 30.degree. out-of-phase (in both a physical sense and a time sense) with respect to the field generated by the relatively lower inductive reactance run winding. When the run and start windings are energized, the geometric and time phase relationship between magnetic fields generated by the run and start windings, and the magnetization of the rotor, cause the rotor to begin rotating from a standstill condition. Once the rotor has sufficient torque to attain its normal running speed, the start winding is "cut-out" of the motor circuit so that the out-of time phase geometrically spaced magnetic field generated by the start winding does not adversely impact motor operation.
Start and run capacitors sometimes are utilized in two capacitor or capacitor-start-capacitor-run motors to change the time phase relationship between the magnetic fields generated by the run and start or auxiliary windings. A start capacitor connected in series circuit with a start or auxiliary winding causes the magnetic field generated by the start winding to be, for example, about 90.degree. (rather than about 30.degree.) out-of-time with the run winding field. As compared to a 30.degree. time phase shift, a 90.degree. time phase shift of the start winding magnetic field results in a higher starting torque, which is desirable in some applications.
Also, rather than switching out the start winding once sufficient rotor torque is attained, the start winding can be utilized as a auxiliary run winding after motor start-up if a run capacitor is connected in series circuit with the start winding. Particularly, the start winding and run capacitor remain in the motor circuit. Such a configuration results in better motor efficiency and power factor.
A motor starting switch may be employed to control the energization and de-energization of the motor start winding or start capacitor connection with an auxiliary winding. A positive temperature coefficient resistor (PTCR), for example, may be used to perform this switching function. Such devices have been used particularly for many compressor motor applications.
A typical PTCR has a low resistance when cool but has an extremely high resistance when hot. The PTCR is connected, for example, in series circuit with the start winding. The temperature/resistance characteristic of the PTCR is selected so that the PTCR has a high resistance once the motor attains its normal running speed. Such a configuration provides the result that the start winding is substantially disassociated from the motor power supply after motor start-up. Of course, if a run or a start capacitor, or both, are coupled to the motor, alternative electrical connections can be made between the PTCR, start winding and capacitors.
Due to the temperature/resistance characteristics of known PTCRs, and as explained above, the PTCR must be maintained at the high temperature in order to maintain the start winding substantially disassociated from the motor power supply after motor start-up. Maintaining the PTCR at the high temperature, however, results in the consumption of energy which is lost as heat.
An object of the present invention is to provide a combination module for a motor protector and a motor starting device which includes a PTCR but substantially eliminates the requirement that the PTCR be maintained at a high temperature for the motor start winding to remain substantially disassociated from the motor power supply after motor start-up.